Reciprocal channel sounding reference signal allocation and configuration

ABSTRACT

Systems and techniques are disclosed to enhance the efficiency of available bandwidth between UEs and base stations. A UE transmits a sounding reference signal to the base station, which characterizes the uplink channel based on the SRS received and, using reciprocity, applies the channel characterization for the downlink channel. The base station may form the beam to the UE based on the uplink channel information obtained from the SRS. As the downlink channel changes the base station needs updated information to maintain its beamforming, meaning it needs a new SRS. Transmission of the SRS takes resources; to minimize this, the UE or the base station can determine a period during which the downlink channel will predictably remain coherent and set up a schedule for sending SRS. Alternatively, the UE or the base station can determine on demand that the channel is losing coherence and initiate an on demand SRS.

CROSS REFERENCE TO RELATED APPLICATIONS

The present application claims priority to and the benefit of the U.S.Provisional Patent Application No. 62/133,328, filed Mar. 14, 2015,which is hereby incorporated by reference in its entirety.

TECHNICAL FIELD

This application relates to wireless communication systems, and moreparticularly to using channel state information obtained from an uplinksounding signal in non-orthogonal or orthogonal applications to beamformdownlink messages to targeted recipients.

BACKGROUND

A wireless communication network may include a number of base stationsthat can support communication for a number of user equipments (UEs). Inrecent years, the carrier frequencies at which base stations and UEscommunicate have continued to increase and include larger bandwidths. Totake advantage of these higher frequencies, more antennas in the samephysical space have been used. For these higher frequency bands to beuseful and approximate the same coverage radius as prior technologies(such as 2G, 3G, or 4G), however, more beam forming gain (and moreaccurate) is becoming necessary.

Further, conventional systems employ various types of reference signals,with varying fixed structures, to provide sufficient measurements andestimations for adaptive multi-antenna operation in uplink and/ordownlink directions. For example, a channel state information referencesignal (CSI-RS) may be used on a downlink from the base station to aidthe base station in beam form determination, an uplink demodulationreference signal (DM-RS) specific to each UE may be used to estimatechannel information for the uplink specifically, and each UE may use asounding reference signal (SRS) on the uplink to aid in scheduling(e.g., determining which frequency bands are good or bad for data).There is no single signal that is able to achieve all of abovefunctionality for UEs.

Reciprocity describes the ability for a station to use information (suchas a multipath delay profile) from one channel (e.g., the uplink) inmaking determinations regarding another channel (e.g., the downlink).Reciprocity has not been available for cellular networks because currentapproaches require reference signals specific for particular antennas,such as CSI-RS in the long term evolution (LTE) context. Further, CSI-RSand other types of signals do not scale well, which is becoming anever-increasing issue as the demand for mobile broadband continues toincrease.

SUMMARY

In one aspect of the disclosure, a method for communicating with a basestation includes determining, at a user equipment (UE), channelcorrelation information for a channel between the UE and the basestation; defining, at the UE, a periodicity of transmission for asounding reference signal (SRS) based on the channel correlationinformation; and transmitting, from the UE, the SRS in accordance withthe defined periodicity.

In an additional aspect of the disclosure, a method includesdetermining, at a base station, channel correlation information for achannel between the BS and a user equipment (UE); transmitting, from thebase station, a request for a sounding reference signal (SRS) from theUE based on the determined channel correlation information; receiving,at the base station, the requested SRS; and training, at the basestation, beamforming to the UE based on the received SRS.

In an additional aspect of the disclosure, a method includestransmitting, from a base station, a request for a sounding referencesignal (SRS) from a user equipment (UE), wherein the request for the SRSincludes configuration information about the SRS; and receiving, at thebase station, an SRS from the UE.

In an additional aspect of the disclosure, a method of communicatingwith a base station includes receiving, at a user equipment (UE), arequest for a sounding reference signal (SRS) from the base station,wherein the request for the SRS includes configuration information aboutthe SRS; and transmitting, from the UE, an SRS based on the received SRSconfiguration information to the base station.

In an additional aspect of the disclosure, a method of communicatingwith a base station includes determining, at a user equipment (UE), aprocessing gain (PG) to communicate with the base station; determining,at the UE, a minimum length of a sounding reference signal (SRS) basedon the determined PG; and broadcasting, from the UE, an SRS having atleast the minimum length to the base station.

In an additional aspect of the disclosure, a method of communicatingwith a wireless network includes determining, at a base station, aprocessing gain (PG) to communicate with a user equipment (UE);determining, at the base station, a minimum length of a soundingreference signal (SRS) based on the determined PG; and transmitting,from the base station, a request to the UE for an SRS having at leastthe minimum length.

In an additional aspect of the disclosure, a user equipment includes aprocessor configured to determine channel correlation information for achannel between the UE and a base station and define a periodicity oftransmission for a sounding reference signal (SRS) based on the channelcorrelation information; and a transceiver configured to transmit theSRS in accordance with the defined periodicity.

In an additional aspect of the disclosure, a base station includes aprocessor configured to determine channel correlation information for achannel between the BS and a user equipment (UE); and a transceiverconfigured to transmit a request for a sounding reference signal (SRS)from the UE based on the determined channel correlation information andreceive the requested SRS, wherein the processor is further configuredto: beamform to the UE based on the received SRS.

In an additional aspect of the disclosure, a base station includes aprocessor configured to generate a request for a sounding referencesignal (SRS) from a user equipment (UE), wherein the request for the SRSincludes configuration information about the SRS; and a transceiverconfigured to transmit the request and receive an SRS from the UE inresponse to the request.

In an additional aspect of the disclosure, a user equipment (UE)includes a transceiver configured to receive, from a base station, arequest for a sounding reference signal (SRS), wherein the request forthe SRS includes configuration information about the SRS; and aprocessor configured to generate the SRS based on the received SRSconfiguration information, wherein the transceiver is further configuredto transmit the generated SRS to the base station.

In an additional aspect of the disclosure, a user equipment includes aprocessor configured to determine a processing gain (PG) to communicatewith a base station and a minimum length of a sounding reference signal(SRS) based on the determined PG; and a transceiver configured tobroadcast an SRS having at least the minimum length to the base station.

In an additional aspect of the disclosure, a base station includes aprocessor configured to determine a processing gain (PG) to communicatewith a user equipment (UE) and a minimum length of a sounding referencesignal (SRS) based on the determined PG; and a transceiver configured totransmit a request to the UE for an SRS having at least the minimumlength.

In an additional aspect of the disclosure, a computer-readable mediumhaving program code recorded thereon includes program code comprisingcode for causing a user equipment (UE) to determine channel correlationinformation for a channel between the UE and a base station; code forcausing the UE to define a periodicity of transmission for a soundingreference signal (SRS) based on the channel correlation information; andcode for causing the UE to transmit the SRS in accordance with thedefined periodicity.

In an additional aspect of the disclosure, a computer-readable mediumhaving program code recorded thereon includes program code comprisingcode for causing a base station to determine channel correlationinformation for a channel between the base station and a user equipment(UE); code for causing the base station to transmit a request for asounding reference signal (SRS) from the UE based on the determinedchannel correlation information; code for causing the base station toreceive the requested SRS; and code for causing the base station tobeamform to the UE based on the received SRS.

In an additional aspect of the disclosure, a computer-readable mediumhaving program code recorded thereon includes program code comprisingcode for causing a base station to transmit a request for a soundingreference signal (SRS) from a user equipment (UE), wherein the requestfor the SRS includes configuration information about the SRS; and codefor causing the base station to receive an SRS from the UE.

In an additional aspect of the disclosure, a computer-readable mediumhaving program code recorded thereon includes program code comprisingcode for causing a user equipment (UE) to receive a request for asounding reference signal (SRS) from the base station, wherein therequest for the SRS includes configuration information about the SRS;and code for causing the UE to transmit an SRS based on the received SRSconfiguration information to the base station.

In an additional aspect of the disclosure, a computer-readable mediumhaving program code recorded thereon includes program code comprisingcode for causing a user equipment (UE) to determine a processing gain(PG) to communicate with a base station; code for causing the UE todetermine a minimum length of a sounding reference signal (SRS) based onthe determined PG; and code for causing the UE to broadcast an SRShaving at least the minimum length to the base station.

In an additional aspect of the disclosure, a computer-readable mediumhaving program code recorded thereon includes program code comprisingcode for causing a base station to determine a processing gain (PG) tocommunicate with a user equipment (UE); code for causing the basestation to determine a minimum length of a sounding reference signal(SRS) based on the determined PG; and code for causing the base stationto transmit a request to the UE for an SRS having at least the minimumlength.

In an additional aspect of the disclosure, a user equipment (UE)includes means for determining channel correlation information for achannel between the UE and a base station; means for defining aperiodicity of transmission for a sounding reference signal (SRS) basedon the channel correlation information; and means for transmitting theSRS in accordance with the defined periodicity.

In an additional aspect of the disclosure, a base station includes meansfor determining channel correlation information for a channel betweenthe base station and a user equipment (UE); means for transmitting arequest for a sounding reference signal (SRS) from the UE based on thedetermined channel correlation information; means for receiving therequested SRS; and means for training beamforming to the UE based on thereceived SRS.

In an additional aspect of the disclosure, a base station includes meansfor transmitting a request for a sounding reference signal (SRS) from auser equipment (UE), wherein the request for the SRS includesconfiguration information about the SRS; and means for receiving an SRSfrom the UE.

In an additional aspect of the disclosure, a user equipment (UE)includes means for receiving a request for a sounding reference signal(SRS) from a base station, wherein the request for the SRS includesconfiguration information about the SRS; and means for transmitting anSRS based on the received SRS configuration information to the basestation.

In an additional aspect of the disclosure, a user equipment (UE)includes means for determining a processing gain (PG) to communicatewith a base station; means for determining a minimum length of asounding reference signal (SRS) based on the determined PG; and meansfor broadcasting an SRS having at least the minimum length to the basestation.

In an additional aspect of the disclosure, a base station includes meansfor determining a processing gain (PG) to communicate with a userequipment (UE);

means for determining a minimum length of a sounding reference signal(SRS) based on the determined PG; and means for transmitting a requestto the UE for an SRS having at least the minimum length.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a wireless communication network in accordance withvarious aspects of the present disclosure.

FIG. 2 illustrates a wireless communication network which uses soundingreference signals to enable beamforming at a base station.

FIG. 3 illustrates an exemplary subframe structure.

FIG. 4 illustrates an exemplary frame structure for a synchronoussubframe system with periodic channel decorrelation.

FIG. 5 illustrates an exemplary frame structure for a synchronoussubframe system with random channel decorrelation.

FIG. 6 illustrates an exemplary subframe structure for multiplexed SRSfrom a multi-antenna user equipment.

FIG. 7 illustrates an exemplary frame structure for an extended lengthSRS in a low-interference environment.

FIG. 8 illustrates an exemplary frame structure for an extended lengthSRS in a high-interference environment.

FIG. 9 is a flowchart illustrating an exemplary method 900 for using anuplink sounding reference signal for channel estimation in accordancewith various aspects of the present disclosure.

FIG. 10 illustrates an exemplary method for using periodic transmissionof an SRS to perform channel estimation in accordance with variousaspects of the present disclosure.

FIG. 11 illustrates an exemplary method for using on demand transmissionof a sounding reference signal to perform channel estimation inaccordance with various aspects of the present disclosure.

FIG. 12 illustrates an exemplary embodiment of a method for using asounding reference signal configured in a specifically desired mannerfor channel estimation in accordance with various aspects of the presentdisclosure.

FIG. 13 illustrates an exemplary method for using a sounding referencesignal configured in a specifically desired manner for channelestimation in accordance with various aspects of the present disclosure.

FIG. 14 illustrates an exemplary method for channel estimation underpoor channel conditions.

FIG. 15 illustrates an exemplary method for channel estimation underpoor channel conditions.

FIG. 16 is a block diagram of an exemplary wireless communicationdevice, such as a user equipment, according to embodiments of thepresent disclosure.

FIG. 17 is a block diagram of an exemplary wireless communicationdevice, such as a base station, according to embodiments of the presentdisclosure.

DETAILED DESCRIPTION

The detailed description set forth below, in connection with theappended drawings, is intended as a description of variousconfigurations and is not intended to represent the only configurationsin which the concepts described herein may be practiced. The detaileddescription includes specific details for the purpose of providing athorough understanding of the various concepts. However, it will beapparent to those skilled in the art that these concepts may bepracticed without these specific details. In some instances, well-knownstructures and components are shown in block diagram form in order toavoid obscuring such concepts.

The techniques described herein may be used for various wirelesscommunication networks such as CDMA, TDMA, FDMA, OFDMA, SC-FDMA andother networks. The terms “network” and “system” are often usedinterchangeably. A CDMA network may implement a radio technology such asUniversal Terrestrial Radio Access (UTRA), cdma2000, etc. UTRA includesWideband CDMA (WCDMA) and other variants of CDMA. cdma2000 coversIS-2000, IS-95 and IS-856 standards. A TDMA network may implement aradio technology such as Global System for Mobile Communications (GSM).An OFDMA network may implement a radio technology such as Evolved UTRA(E-UTRA), Ultra Mobile Broadband (UMB), IEEE 802.11 (Wi-Fi), IEEE 802.16(WiMAX), IEEE 802.20, Flash-OFDMA, etc. UTRA and E-UTRA are part ofUniversal Mobile Telecommunication System (UMTS). 3GPP Long TermEvolution (LTE) and LTE-Advanced (LTE-A) are new releases of UMTS thatuse E-UTRA. UTRA, E-UTRA, UMTS, LTE, LTE-A and GSM are described indocuments from an organization named “3rd Generation PartnershipProject” (3GPP). CDMA2000 and UMB are described in documents from anorganization named “3rd Generation Partnership Project 2” (3GPP2). Thetechniques described herein may be used for the wireless networks andradio technologies mentioned above as well as other wireless networksand radio technologies, such as a next generation (e.g., 5^(th)Generation (5G)) network.

Embodiments of the present disclosure introduce systems and techniquesto enhance the efficiency of use of available bandwidth in wirelesscommunications channels between UEs 102 and wireless base stations 104.In an embodiment, multiplexing may be used to aid in increasing theefficiency of use of channel resources, such as frequency divisionmultiple access (FDMA), time division multiple access (TDMA), codedivision multiple access (CDMA), or spatial division multiple access(SDMA). One way of achieving SDMA, or space division multiplexing, is byuse of beamforming. If a device has multiple antennas, it may transmitsignals from all antennas at once while altering the phase of the signalfrom each antenna to produce constructive and destructive interference.The interference may be calibrated to produce constructive interferencein a specific direction and destructive interference in all otherdirections, thus essentially transmitting a “beam” of information thatdoes not create interference in any other spatial area. Multiple beamsmay therefore be transmitted at once in different directions withoutinterference. In order to successfully beamform, the multiple antennadevice uses information about the channel between itself and itsintended recipient device to create a beam which will reach therecipient.

Thus, according to embodiments of the present disclosure, a base stationmay harness channel reciprocity in order to use channel informationobtained from the uplink channel from a UE to the base station for thedownlink. A UE may transmit a sounding reference signal (SRS) to thebase station, and the base station, in turn, may characterize the uplinkchannel based on the SRS received and, using reciprocity, apply the samechannel characterization for the downlink channel back to the UE. Aspart of applying the channel information to the downlink, the basestation may form the beam to the UE based on the uplink channelinformation obtained from the SRS. However, as the downlink channelchanges the base station needs updated information to maintain itsbeamforming, which means it needs a new SRS. Transmission of the SRStakes resources, and it is desirable to minimize the amount of SRS sent.In some embodiments it is possible for the UE or the base station todetermine a period during which the downlink channel will predictablyremain coherent, and thus set up a periodic schedule for sending SRS forthe base station retrain its beamforming. In other embodiments it ispossible for the UE or the base station to determine on demand that thechannel is losing coherence, and thus initiate an on demand SRS toretrain beamforming at the base station.

In some embodiments of the present disclosure, the UE or base stationmay determine that channel conditions are poor. In this case, anelongated SRS may be necessary to fully characterize the uplink channel.In some embodiments the UE may send an elongated SRS in one continuousburst, while in other embodiments the UE may fragment the SRS and sendit in multiple bursts so as to avoid interfering with othercommunications.

FIG. 1 illustrates a wireless communication network 100 in accordancewith various aspects of the present disclosure. The wirelesscommunication network 100 may include a number of UEs 102, as well as anumber of base stations 104. The base stations 104 may include anevolved Node B (eNodeB). A base station may also be referred to as abase transceiver station, a node B, or an access point. A base station104 may be a station that communicates with the UEs 102 and may also bereferred to as a base station, a node B, an access point, and the like.

The base stations 104 communicate with the UEs 102 as indicated bycommunication signals 106. A UE 102 may communicate with the basestation 104 via an uplink and a downlink. The downlink (or forward link)refers to the communication link from the base station 104 to the UE102. The uplink (or reverse link) refers to the communication link fromthe UE 102 to the base station 104. The base stations 104 may alsocommunicate with one another, directly or indirectly, over wired and/orwireless connections, as indicated by communication signals 108.

UEs 102 may be dispersed throughout the wireless network 100, as shown,and each UE 102 may be stationary or mobile. The UE 102 may also bereferred to as a terminal, a mobile station, a subscriber unit, etc. TheUE 102 may be a cellular phone, a smartphone, a personal digitalassistant, a wireless modem, a laptop computer, a tablet computer, etc.The wireless communication network 100 is one example of a network towhich various aspects of the disclosure apply.

Each base station 104 may provide communication coverage for aparticular geographic area. In 3GPP, the term “cell” can refer to thisparticular geographic coverage area of a base station and/or a basestation subsystem serving the coverage area, depending on the context inwhich the term is used. In this regard, a base station 104 may providecommunication coverage for a macro cell, a pico cell, a femto cell,and/or other types of cell. A macro cell generally covers a relativelylarge geographic area (e.g., several kilometers in radius) and may allowunrestricted access by UEs with service subscriptions with the networkprovider. A pico cell may generally cover a relatively smallergeographic area and may allow unrestricted access by UEs with servicesubscriptions with the network provider. A femto cell may also generallycover a relatively small geographic area (e.g., a home) and, in additionto unrestricted access, may also provide restricted access by UEs havingan association with the femto cell (e.g., UEs in a closed subscribergroup (CSG), UEs for users in the home, and the like). A base stationfor a macro cell may be referred to as a macro base station. A basestation for a pico cell may be referred to as a pico base station. Abase station for a femto cell may be referred to as a femto base stationor a home base station.

In the example shown in FIG. 1, the base stations 104 a, 104 b and 104 care examples of macro base stations for the coverage areas 110 a, 110 band 110 c, respectively. The base stations 104 d and 104 e are examplesof pico and/or femto base stations for the coverage areas 110 d and 110e, respectively. As will be recognized, a base station 104 may supportone or multiple (e.g., two, three, four, and the like) cells.

The wireless network 100 may also include relay stations. A relaystation is a station that receives a transmission of data and/or otherinformation from an upstream station (e.g., a base station, a UE, or thelike) and sends a transmission of the data and/or other information to adownstream station (e.g., another UE, another base station, or thelike). A relay station may also be a UE that relays transmissions forother UEs. A relay station may also be referred to as a relay basestation, a relay UE, a relay, and the like.

The wireless network 100 may support synchronous or asynchronousoperation. For synchronous operation, the base stations 104 may havesimilar frame timing, and transmissions from different base stations 104may be approximately aligned in time. For asynchronous operation, thebase stations 104 may have different frame timing, and transmissionsfrom different base stations 104 may not be aligned in time.

In some implementations, the wireless network 100 utilizes orthogonalfrequency division multiplexing (OFDM) on the downlink andsingle-carrier frequency division multiplexing (SC-FDM) on the uplink.OFDM and SC-FDM partition the system bandwidth into multiple (K)orthogonal subcarriers, which are also commonly referred to as tones,bins, or the like. Each subcarrier may be modulated with data. Ingeneral, modulation symbols are sent in the frequency domain with OFDMand in the time domain with SC-FDM. The spacing between adjacentsubcarriers may be fixed, and the total number of subcarriers (K) may bedependent on the system bandwidth. For example, K may be equal to 72,180, 300, 600, 900, and 1200 for a corresponding system bandwidth of1.4, 3, 5, 10, 15, or 20 megahertz (MHz), respectively. The systembandwidth may also be partitioned into sub-bands. For example, asub-band may cover 1.08 MHz, and there may be 1, 2, 4, 8 or 16 sub-bandsfor a corresponding system bandwidth of 1.4, 3, 5, 10, 15, or 20 MHz,respectively.

Referring now to FIG. 2, there is shown an example of a system that maybe used to enhance the efficiency of use of available bandwidth inwireless communications channels between one or more UEs 102 and one ormore base stations 104, as discussed above with respect to FIG. 1. FIG.2 illustrates one base station 104 and one UE 102 for purposes ofsimplicity of discussion, though it will be recognized that embodimentsof the present disclosure may scale to many more UEs 102 and/or basestations 104. The UE 102 and the base station 104 may communication witheach other at various frequencies. For example, in one embodiment the UE102 and the base station 104 may communicate at sub-6 GHz frequencies,while in another embodiment at above 6 GHz frequencies, to name just twoexamples.

UE 102 broadcasts a sounding reference signal (SRS) 202 that is receivedby base station 104. In an embodiment, the SRS 202 may be anomni-directional transmission, while in another embodiment the SRS 202may be a wide-beam transmission. Upon receipt of the SRS 202, the basestation 104 is able to gather from the SRS 202, either explicitly orimplicitly, channel information for the uplink channel between the UE102 and the base station 104. The base station 104 may then use thatuplink channel information to train its antennas to beamform a downlink204 to the same UE 102.

To derive the most advantage from reciprocity (applying channelinformation obtained from the SRS 202 in the uplink), the base station104 may rapidly re-apply that information (by training) for beamforming(or focusing) a downlink transmission to the UE 102 so as to minimizethe effects of channel decorrelation. To assist in therapid-reapplication of the channel information in the downlink,embodiments of the present disclosure utilize a short subframestructure. Referring now to FIG. 3, an exemplary subframe structure 300is illustrated that operates within a short timeframe so as to minimizethe effects of decorrelation in the channel. In an embodiment, the shorttimeframe may be approximately 500 microseconds, though it may also beshorter or longer than that. The short timeframe allows the base station104 to essentially “freeze” the channel state for the duration of thesubframe, during which the base station 104 may train and form the beamfor the downlink and then provide a downlink burst.

Communications between UE 102 and base station 104 can be divided in thetime domain into subframes (SFs) 300, such as the SF 300 illustrated inFIG. 3. A single subframe is illustrated in FIG. 3 for ease ofillustration; as will be recognized, the structure of the SF 300 isscalable to any number of subframes as necessary or desired. Each SF 300is divided into an uplink (UL) portion 302 and a downlink (DL) portion304, separated by a transition portion U/D. As part of the UL portion302, the UE 102 may send various types of signals to the base station104. These may include, for example, an SRS (used here for transmitbeamforming at the base station and in place of the uplink DMRS), uplinkdata, and optionally requests for information. The transition portionU/D is provided between the UL portion 302 and the DL portion 304.During the DL portion, the base station 104 sends various types ofsignals to the UE 102, including for example a user-equipment referencesignal (UERS) and downlink data (e.g., in a downlink burst).

In some embodiments, the base station 104 may use the SRS in the ULportion 302 derive multiple pieces of information that facilitate thedownlink between the UE 102 and the base station 104. For example, basedon the SRS the base station 104 having multiple antennas is able totrain its antennas to beamform the DL data transmitted back to the UE102 so that, for instance, interference with other wirelesscommunication devices in the range of the base station 104 is reduced.Beamforming relies on information about the channel between the UE 102and the base station 104 that the base station 104 derives from theuplink SRS and then applies to the downlink based on reciprocity. Thebase station 104 can retrain its antennas as the channel changes overtime (e.g., periodically or randomly), for example according tosubsequent SRS received from the UE 102. This may happen, for example,if the UE 102 is moving or if other moving objects enter or leave thearea/interefere with the uplink (or downlink) channel. According toembodiments of the present disclosure, the subframe 300 is provided aspart of a synchronous system, such that the subframe 300 is providedrepeatedly over time so that the base station 104 may retrain the beamsto accommodate for UE 102 motion and channel decorrelation related tothat movement (and/or other influences).

Channel reciprocity may allow the base station 104 to apply informationabout the channel in the UL direction to estimate one or more channelproperties in the DL direction, which can be used to beamform the DLtransmissions. In this manner, the base station 104 can train itsantennas based on the SRS from the UE 102. The SRS may further includeinformation that allows the base station 104 to demodulate data receivedfrom the UE 102 during the UL portion of the SF 300. The base station104 may additionally determine, from the SRS, scheduling informationthat allows the base station 104 to schedule future SFs 300 (e.g.,frequency bands, etc.) for communicating with the UE 102.

In some embodiments, a synchronous SF system allows for interferencemanagement by allocating how often SRSs are sent by the UE 102.Referring now to FIG. 4, an embodiment is shown in which the channelbetween the UE 102 and the base station 104 may be changing slowly, suchas when the UE 102 is relatively stationary. In this case, the UE 102 orthe base station 104 may determine that the base station's 104 currentantenna beamforming will allow for acceptable communications for xnumber of SFs 300 before it needs to be retrained. For example, thisperiod may be every two SFs (as illustrated by frame structure 400),every three, every four, or more SFs to name just a few examples. Inthis case, the base station 104 may instruct the UE 102, or the UE 102may determine for itself, to only allocate a portion of the UL for anSRS once every x number of SFs. This allows the base station 104 toretrain its beamforming only when necessary to maintain the channel atan acceptable quality level. In other instances, the SRS can be sentevery SF (e.g., even when the channel is changing slowly).

Referring now to FIG. 5, another embodiment of a synchronous SF systemis shown in which the base station 104 may have a schedule forcommunicating with multiple UEs 102 during designated SFs. In anembodiment, the base station 104 may determine that it only needs toretrain its antenna beamforming for a particular UE 102 occasionally.For example, as illustrated in FIG. 5, there may be two UEs 102scheduled to communicate during one SF 500 (corresponding to user 1 anduser 2, respectively, and SRS 1 and SRS 2, respectively). When the SF500 arrives during which the base station 104 is scheduled tocommunicate with the first and second UEs 102, the base station 104allocates a first UL portion of the SF 500 to an SRS from the first UE102 (e.g., SRS 1) and a second UL portion of the SF 500 to an SRS fromthe second UE 102 (e.g., SRS 2). The first and second UEs 102 send firstand second SRSs, respectively, in response. This allows the base station104 to train its antenna beamforming to compensate for the DL channelsto each UE 102 as they then exist. The requests for SRSs are made duringa DL portion of the SF 500 inserted at the beginning of the SF 500,illustrated in FIG. 5 as existing prior to the UL portion of the SF 500.

In some embodiments, multiplexing may be used to allow the base station104 to communicate with multiple UEs 102 during the DL portion of oneSF. One advantage of beamforming is that it allows the base station 104to make use of space division multiplexing alongside other types ofmultiplexing such as frequency division multiplexing and code divisionmultiplexing. The base station 104 may therefore request that multipleUEs 102 send an SRS during one SF, allowing the base station 104 toretrain its antenna beamforming for each UE 102 that the base station104 will communicate with during that SF. In order to facilitate channelreciprocity, an SRS may be configured as a broadcast signal using theentire channel frequency bandwidth. The base station 104 may thereforeinform each UE 102 to send its respective SRS during non-conflictingportions of the UL portion of the SF 300 and/or the UEs 102 may beinstructed to use code division multiplexing or the like to avoidcollision of their respective SRS at the base station 104.

In some embodiments, a request for an SRS from the base station 104 mayprovide further details to the UE 102 regarding the structure of the SRSthat the UE 102 should use and the manner of transmission of the SRSfrom the UE 102. For example, the request may instruct the UE 102 to usea specific length of SRS to account for channel conditions, as describedfurther below with respect to FIG. 7. The request may also (oralternatively) instruct the UE 102 to use a specific sub-band for its ULcommunication. The request may further (or alternatively) specify whichphysical resources the UE 102 is to use during the data upload in orderto multiplex with other UEs 102 in the same SF. The request may also (oralternatively) specify whether a UE 102 having multiple antennas shouldsend its UL data simultaneously from multiple antennas or multiplex itsantennas.

Referring now to FIG. 6, there is shown an illustration of a SF 600 usedby a UE 102 with two antennas sending multiplexed SRS, one from eachantenna in adjacent blocks of time within the UL portion of the SF 600.The base station 104 may send information regarding the structure andmanner of sending of the SRS to wireless communication devices withscheduled or periodic SRS allocation, such as a UE 102.

In some embodiments, either the UE 102 or the base station 104 maydetermine a minimum processing gain (PG) needed to compensate for a poorchannel, for example when a UE 102 is distant from a base station 104.The UE 102 may determine a minimum PG by monitoring how long it takes tosuccessfully receive a SYNC signal from the base station 104. The basestation 104 may determine a minimum PG by monitoring how long it takesto set up a random access channel (RACH) with the UE 102. In order toachieve the minimum PG, the length of the SRS may need to be scaled toexceed the portion of the SF allocated to UL.

In some embodiments, the base station 104 may be isolated fromneighboring base stations 104. As a result, there may be little concernfor interference with neighboring base stations 104 and/or otherwireless communication devices. In this case, it is not critical to keeptransmissions limited to one SF to avoid interference during a futureSF.

Referring now to FIG. 7, there is illustrated a frame structure 700wherein the SRS length is scaled to the length necessary to achieve theminimum PG. In the illustrated embodiment, the SRS extends across morethan one entire SF. The base station 104 may schedule transmissions withmultiple UEs 102 to allow them the time needed to completecommunications at the minimum PG. Alternatively, the UEs 102 or the basestation 104 may determine a period that would allow each UE 102 the timeneeded to complete communications to achieve a desired PG.

In some embodiments, the base station 104 may be embedded amongneighboring base stations 104 such that the transmission ranges of thebase stations 104 overlap. In this case, it may be desirable to keeptransmissions limited to within one SF so as to avoid causinginterference with neighboring communications.

Referring now to FIG. 8, a frame structure 800 is illustrated wherein anelongated SRS necessary to achieve a desired PG is divided among theallocated UL period of multiple SFs while maintaining phase continuity,so that the base station 104 may coherently combine the fragmented SRSportions together to form a single combined SRS. As can be seen, incontrast to the extended SRS in FIG. 7 that extends across more than oneSF, in FIG. 8 the extended SRS is divided up so that no one SF is fullycovered by an SRS.

In some embodiments, when the base station 104 determines that anelongated SRS is necessary as described above, the base station 104 maysignal to the UE 102 during a DL portion of a SF to increase the lengthof its SRS. For example, the base station 104 may signal to the UE 102to send an SRS of a particular length (e.g., length y) or greater. Inother embodiments, when the UE 102 determines that a longer SRS isnecessary as described above, the UE 102 may signal to the base station104 to be prepared to receive a longer SRS, such as an SRS of aparticular length (e.g., length y) or greater.

FIG. 9 is a flowchart illustrating an exemplary method 900 for using anuplink sounding reference signal for channel estimation in accordancewith various aspects of the present disclosure. The method 900 may beimplemented in base station 104. The method 900 will be described withrespect to a single base station 104 for simplicity of discussion,though it will be recognized that the aspects described herein may beapplicable to any number of base stations 104. It is understood thatadditional method blocks can be provided before, during, and after theblocks of method 900, and that some of the blocks described can bereplaced or eliminated for other embodiments of the method 900.

At block 902, a base station 104 receives an SRS from a UE 102 in anuplink communication, as described according to the various embodimentsabove. For example, the base station 104 may receive the SRS as part ofan uplink portion of a subframe as illustrated in FIG. 3. According tothe various embodiments of the present disclosure, the base station 102may receive a single SRS from a single-antenna UE 102, multiple SRScorresponding to multiple antennas of a single UE 102, multiple SRScorresponding to single antennas of multiple UEs 102, and/or multipleSRS corresponding to multiple antennas of multiple UEs 102. Further, theSRS may be provided to the base station 104 according to non-orthogonalor orthogonal SRS, depending upon embodiment.

At block 904, the base station 104 extracts information about the uplinkfrom the SRS received at block 902. This may include information usefulin demodulating uplink data including in the uplink portion of thesubframe, scheduling information, and channel information about theuplink channel.

At block 906, the base station 104 schedules the downlink communication(e.g., the downlink burst that is part of the downlink portion of asubframe), based on information extracted from the SRS at block 904.

At block 908, the base station 104 trains the beamforming for the one ormore antennas of the base station 104 based on channel informationextracted from the SRS received from the UE 102. Based on the SRS, thebeamforming may be invariant to the number of antennas within thesystem, rendering embodiments of the present disclosureforward-compatible with future technologies that include more antennas(e.g., 16, 32, etc.) in MIMO arrays for example.

At block 910, as part of the same subframe, the base station 104transmits a downlink burst including one or more reference signals (suchas a UERS) as well as downlink data. With the beam forms of the antennasof the base station 104 trained based on the channel information derivedfrom the uplink SRS, applied to the downlink by taking advantage ofreciprocity during a short timeframe encapsulated by the subframe, thebase station 104 is able to better improve its utilization of higherfrequencies while still providing a substantially equivalent range thatis possible with lower frequencies/evolution technologies (2G, 3G, 4Gfor example).

It is understood that method 900 may be implemented in program codestored on a computer readable medium. The program code may, for example,cause a processor to implement the blocks 902-910 upon reading the codefrom the computer readable medium. In some embodiments, the base station104 of the present disclosure may include such a processor and such acomputer readable medium with program code stored in it.

FIG. 10 illustrates an exemplary method 1000 for using periodictransmission of an SRS to perform channel estimation in accordance withvarious aspects of the present disclosure. Channel estimation via SRS,due to channel reciprocity, allows a base station 104 to retrain and/orupdate its beamforming. The method 1000 may be implemented in a UE 102.It is understood that additional method blocks can be provided before,during, and after the blocks of method 1000, and that some of the blocksdescribed can be replaced or eliminated for other embodiments of themethod 1000.

At block 1002, the UE 102 determines how quickly the UL channel betweenthe UE 102 and the base station 104 is changing, and thus how quicklythe channel is decorrelating.

At block 1004, the UE 102 uses the channel correlation information todetermine a period after which the channel will be decorrelated. The UE102 need only send an SRS to the base station 104 once every period inorder to retrain and/or update the beamforming.

Therefore, at block 1006, the UE 102 transmits an SRS once every periodto facilitate recorrelation by the base station 104. In every period,the SRS is transmitted during a designated portion of the UL portion ofa subframe.

It is understood that method 1000 may be implemented in program codestored on a computer readable medium. The program code may, for example,cause a processor to implement the blocks 1002-1006 upon reading thecode from the computer readable medium. In some embodiments, the UE 102of the present disclosure may include such a processor and such acomputer readable medium with program code stored in it.

Referring now to FIG. 11, a flowchart is illustrated of an exemplarymethod 1100 for using on demand transmission of a sounding referencesignal to perform channel estimation in accordance with various aspectsof the present disclosure. Channel estimation via SRS, due to channelreciprocity, allows a base station 104 to retrain and/or update thebeamforming. The method 1100 may be implemented in a base station 104.It is understood that additional method blocks can be provided before,during, and after the blocks of method 1100, and that some of the blocksdescribed can be replaced or eliminated for other embodiments of themethod 1100.

At block 1102, the base station 104 determines whether the DL channelbetween the base station 104 and the UE 102 is decorrelating.

At block 1104, the base station 104 uses the channel correlationinformation to determine whether it needs to retrain and/or update thebeamforming. If so, the base station 104 sends a request for an SRS toUE 102.

At block 1106, the base station 104 receives the requested SRS and atblock 1108, the base station 104 uses information obtained from thereceived SRS, either explicitly or implicitly, to retrain and/or updatethe beamforming to the UE 102.

It is understood that method 1100 may be implemented in program codestored on a computer readable medium. The program code may, for example,cause a processor to implement the blocks 1102-1108 upon reading thecode from the computer readable medium. In some embodiments, the basestation 104 of the present disclosure may include such a processor andsuch a computer readable medium with program code stored in it.

Referring now to FIG. 12, there is illustrated an exemplary embodimentof a method 1200 for using a sounding reference signal configured in aspecifically desired manner for channel estimation in accordance withvarious aspects of the present disclosure. The method 1200 may beimplemented in a base station 104. It is understood that additionalmethod blocks can be provided before, during, and after the blocks ofmethod 1200, and that some of the blocks described can be replaced oreliminated for other embodiments of the method 1200.

At block 1202, the base station 104 determines a desired configurationfor an and/or manner of sending an SRS which it will request from a UE102. The base station 104 may request that the UE 102 use a particularlength SRS. The base station 104 may alternatively request that the UE102 use a specific sub-band of the channel when sending its SRS. Thebase station 104 may alternatively request that the UE 102 use aspecific configuration of physical resources when sending its SRS. Thebase station 104 may alternatively request that the UE 102 havingmultiple antennas send SRSs simultaneously from all antennas, or that itmultiplex its SRSs from each antenna.

At block 1204, the base station 104 transmits to the UE 102 a requestfor SRS containing the desired configuration and/or manner of sendingfrom block 1202. At block 1206, the base station 104 receives therequested SRS from the UE 102. At block 1208 the base station 104 trainsits beamforming based on information obtained, either explicitly orimplicitly, from the received SRS.

It is understood that method 1200 may be implemented in program codestored on a computer readable medium. The program code may, for example,cause a processor to implement the blocks 1202-1208 upon reading thecode from the computer readable medium. In some embodiments, the UE 102and base station 104 of the present disclosure may include such aprocessor and such a computer readable medium with program code storedin it.

Referring now to FIG. 13, there is illustrated an exemplary method 1300for using a sounding reference signal configured in a specificallydesired manner for channel estimation in accordance with various aspectsof the present disclosure. The method 1300 may be implemented in a UE102. It is understood that additional method blocks can be providedbefore, during, and after the blocks of method 1300, and that some ofthe blocks described can be replaced or eliminated for other embodimentsof the method 1300.

At block 1302, a UE 102 receives a request for an SRS from a basestation 104. The request contains requests from the base station 104that the SRS be configured and/or sent in a specific manner. The requestmay include that the UE 102 use a particular length SRS. The request mayfurther include that the UE 102 use a specific sub-band of the channelwhen sending its SRS. The request may further include that the UE 102use a specific configuration of physical resources when sending its SRS.The request may further include that a UE 102 having multiple antennassend SRSs simultaneously from all antennas, or that it multiplex itsSRSs from each antenna. At block 1304 the SRS transmits an SRSconforming to the requested configuration and/or manner of sending.

It is understood that method 1300 may be implemented in program codestored on a computer readable medium. The program code may, for example,cause a processor to implement the blocks 1302 and 1304 upon reading thecode from the computer readable medium. In some embodiments, the UE 102of the present disclosure may include such a processor and such acomputer readable medium with program code stored in it.

Referring now to FIG. 14, there is illustrated an exemplary method 1400for channel estimation under poor channel conditions. The method 1400may be implemented in a UE 102. It is understood that additional methodblocks can be provided before, during, and after the blocks of method1400, and that some of the blocks described can be replaced oreliminated for other embodiments of the method 1400.

At block 1402, the UE 102 determines whether the channel conditionsbetween the UE 102 and a base station 104 are poor. This may bedetermined, for instance, by monitoring the length of time it takes toreceive a SYNC signal from the base station 104. The UE 102 may use thisinformation to determine a minimum processing gain (PG) required toestablish a useable channel with the base station 104. The UE 102 mayincrease PG by elongating the length of its SRS. At block 1404, the UE102 determines, based on the minimum PG, a minimum length SRS to reachthe minimum PG. At block 1406, the UE 102 transmits the elongated SRS tothe base station 104. The elongated SRS may take up more than theallotted UL portion of a subframe, or even more than one entiresubframe. In some embodiments, the UE 102 transmits the elongated SRScontinuously over as many subframes as needed to reach the minimum PG.In other embodiments, the UE 102 transmits the elongated SRS infragments, each fragment being transmitted only during an allotted ULportion of a subframe.

It is understood that method 1400 may be implemented in program codestored on a computer readable medium. The program code may, for example,cause a processor to implement the blocks 1402-1406 upon reading thecode from the computer readable medium. In some embodiments, the UE 102of the present disclosure may include such a processor and such acomputer readable medium with program code stored in it.

Referring now to FIG. 15, there is illustrated an exemplary method 1500for channel estimation under poor channel conditions. The method 1500may be implemented in a base station 104. It is understood thatadditional method blocks can be provided before, during, and after theblocks of method 1500, and that some of the blocks described can bereplaced or eliminated for other embodiments of the method 1500.

At block 1502, the base station 104 determines whether the channelconditions between the base station 104 and a UE 102 are poor. This maybe determined, for instance, by monitoring the length of time it takesto establish a random access channel (RACH) with the UE 102. The basestation 104 may use this information to determine a minimum processinggain (PG) required to establish a useable channel with the UE 102. PGmay be increased by elongating the length of its SRS sent from the UE102.

At block 1504, the base station 104 determines, based on the minimum PG,a minimum length SRS to reach the minimum PG.

At block 1506, the base station 104 transmits a request for an elongatedSRS to the UE 102. The elongated SRS may take up more than the allottedUL portion of a subframe, or even more than one entire subframe. In someembodiments, the base station 104 receives the elongated SRScontinuously over as many subframes as needed to reach the minimum PG.In other embodiments, the base station 104 receives the elongated SRS infragments, each fragment being transmitted only during an allotted ULportion of a subframe.

It is understood that method 1500 may be implemented in program codestored on a computer readable medium. The program code may, for example,cause a processor to implement the blocks 1502-1506 upon reading thecode from the computer readable medium. In some embodiments, the basestation 104 of the present disclosure may include such a processor andsuch a computer readable medium with program code stored in it.

FIG. 16 is a block diagram of an exemplary wireless communication device1600 according to embodiments of the present disclosure. The wirelesscommunication device 1600 may be a base UE 102 as discussed above. Asshown, the UE 102 may include a processor 1602, a memory 1604, acorrelation information module 1608, a transceiver 1610 (including amodem 1612 and RF unit 1614), and an antenna 1616. These elements may bein direct or indirect communication with each other, for example via oneor more buses.

The processor 1602 may include a central processing unit (CPU), adigital signal processor (DSP), an application-specific integratedcircuit (ASIC), a controller, a field programmable gate array (FPGA)device, another hardware device, a firmware device, or any combinationthereof configured to perform the operations described herein withreference to UEs 102 introduced above with respect to FIG. 1 anddiscussed in more detail above. In particular, the processor 1602 may beutilized in combination with the other components of the UE 102,including correlation information module 1608, to perform the variousfunctions associated with determining whether an update of SRSperiodicity is necessary and what the required minimum length of the SRSis as described in greater detail above. The processor 1602 may also beimplemented as a combination of computing devices, e.g., a combinationof a DSP and a microprocessor, a plurality of microprocessors, one ormore microprocessors in conjunction with a DSP core, or any other suchconfiguration.

The memory 1604 may include a cache memory (e.g., a cache memory of theprocessor 1602), random access memory (RAM), magnetoresistive RAM(MRAM), read-only memory (ROM), programmable read-only memory (PROM),erasable programmable read only memory (EPROM), electrically erasableprogrammable read only memory (EEPROM), flash memory, solid state memorydevice, hard disk drives, other forms of volatile and non-volatilememory, or a combination of different types of memory. In an embodiment,the memory 1604 includes a non-transitory computer-readable medium. Thememory 1604 may store instructions 1606. The instructions 1606 mayinclude instructions that, when executed by the processor 1602, causethe processor 1602 to perform the operations described herein withreference to the UEs 102 in connection with embodiments of the presentdisclosure. Instructions 1606 may also be referred to as code. The terms“instructions” and “code” should be interpreted broadly to include anytype of computer-readable statement(s). For example, the terms“instructions” and “code” may refer to one or more programs, routines,sub-routines, functions, procedures, etc. “Instructions” and “code” mayinclude a single computer-readable statement or many computer-readablestatements.

The correlation information module 1608 may be used for various aspectsof the present disclosure. For example, the correlation informationmodule 1608 may determine the rate of decorrelation of the channel dueto, for example, movement of the UE 102. The correlation informationmodule 1608 may then use the determined rate of decorrelation todetermine a period after which the channel will be decorrelated, and toschedule SRS transmission at the determined period. In anotherembodiment, the correlation information module 1608 may interpret arequest for an SRS from a base station 104 and configure an SRSaccordingly. In another embodiment, the correlation information module1608 may determine a processing gain necessary to communicate with abase station 104. The correlation information module 1608 may then usethe processing gain information to determine a minimum length of SRSnecessary, and configure an SRS accordingly for transmission to the basestation 104

As shown, the transceiver 1610 may include the modem subsystem 1612 andthe radio frequency (RF) unit 1614. The transceiver 1610 can beconfigured to communicate bi-directionally with other devices, such asbase stations 104. The modem subsystem 1612 may be configured tomodulate and/or encode the data from the correlation information module1608 and other aspects of the UE 102, such as processor 1602 and/ormemory 1604, according to a modulation and coding scheme (MCS), e.g., alow-density parity check (LDPC) coding scheme, a turbo coding scheme, aconvolutional coding scheme, etc. The RF unit 1614 may be configured toprocess (e.g., perform analog to digital conversion or digital to analogconversion, etc.) modulated/encoded data from the modem subsystem 1612(on outbound transmissions) or of transmissions originating from anothersource such as a UE 102 or a base station 104. Although shown asintegrated together in transceiver 1610, the modem subsystem 1612 andthe RF unit 1614 may be separate devices that are coupled together atthe UE 102 to enable the UE 102 to communicate with other devices.

The RF unit 1614 may provide the modulated and/or processed data, e.g.data packets (or, more generally, data messages that may contain one ormore data packets and other information), to the antenna 1616 fortransmission to one or more other devices. This may include, forexample, transmission of an SRS according to embodiments of the presentdisclosure. The antenna 1616 may further receive data messagestransmitted from other devices and provide the received data messagesfor processing and/or demodulation at the transceiver 1610. AlthoughFIG. 16 illustrates antenna 1616 as a single antenna, antenna 1616 mayinclude multiple antennas of similar or different designs in order tosustain multiple transmission links.

FIG. 17 illustrates a block diagram of an exemplary base station 104according to the present disclosure. The base station 104 may include aprocessor 1702, a memory 1704, a beamforming module 1708, a transceiver1710 (including a modem 1712 and RF unit 1714), and an antenna 1716.These elements may be in direct or indirect communication with eachother, for example via one or more buses.

The processor 1702 may have various features as a specific-typeprocessor. For example, these may include a CPU, a DSP, an ASIC, acontroller, a FPGA device, another hardware device, a firmware device,or any combination thereof configured to perform the operationsdescribed herein with reference to the base stations 104 introduced inFIG. 1 above. The processor 1702 may also be implemented as acombination of computing devices, e.g., a combination of a DSP and amicroprocessor, a plurality of microprocessors, one or moremicroprocessors in conjunction with a DSP core, or any other suchconfiguration.

The memory 1704 may include a cache memory (e.g., a cache memory of theprocessor 1702), RAM, MRAM, ROM, PROM, EPROM, EEPROM, flash memory, asolid state memory device, one or more hard disk drives, memristor-basedarrays, other forms of volatile and non-volatile memory, or acombination of different types of memory. In some embodiments, thememory 1704 may include a non-transitory computer-readable medium. Thememory 1704 may store instructions 1706. The instructions 1706 mayinclude instructions that, when executed by the processor 1702, causethe processor 1702 to perform operations described herein with referenceto a base station 104 in connection with embodiments of the presentdisclosure. Instructions 1706 may also be referred to as code, which maybe interpreted broadly to include any type of computer-readablestatement(s) as discussed above with respect to FIG. 2.

The beamforming module 1708 may be used for various aspects of thepresent disclosure. For example, the beamforming module 1708 may extractinformation from an SRS received from a UE 102 and train beamforming ateach antenna 1716 based on the extracted information. In anotherembodiment, the beamforming module 1708 may determine correlationinformation for the DL channel to a UE 102 and transmit a request for anSRS to the UE 102 based on the correlation information. In anotherembodiment, the beamforming module 1708 may determine a processing gainnecessary to communicate with a UE 102. The beamforming module 1708 maythen use the processing gain information to determine a minimum lengthof SRS necessary, and transmit a request for the minimum length SRS tothe UE 102.

As shown, the transceiver 1710 may include the modem subsystem 1712 andthe radio frequency (RF) unit 1714. The transceiver 1710 can beconfigured to communicate bi-directionally with other devices, such asUE 102 and/or another core network element. The modem subsystem 1712 maybe configured to modulate and/or encode data according to a MCS, e.g., aLDPC coding scheme, a turbo coding scheme, a convolutional codingscheme, etc. The RF unit 1714 may be configured to process (e.g.,perform analog to digital conversion or digital to analog conversion,etc.) modulated/encoded data from the modem subsystem 1712 (on outboundtransmissions) or of transmissions originating from another source suchas a UE 102. Although shown as integrated together in transceiver 1710,the modem subsystem 1712 and the RF unit 1714 may be separate devicesthat are coupled together at the base station 104 to enable the basestation 104 to communicate with other devices.

The RF unit 1714 may provide the modulated and/or processed data, e.g.data packets (or, more generally, data messages that may contain one ormore data packets and other information), to the antenna 1716 fortransmission to one or more other devices. This may include, forexample, transmission of information to complete attachment to a networkand communication with a camped UE 102 according to embodiments of thepresent disclosure. The antenna 1716 may further receive data messagestransmitted from other devices and provide the received data messagesfor processing and/or demodulation at the transceiver 1710. AlthoughFIG. 17 illustrates antenna 1716 as a single antenna, antenna 1716 mayinclude multiple antennas of similar or different designs in order tosustain multiple transmission links.

Information and signals may be represented using any of a variety ofdifferent technologies and techniques. For example, data, instructions,commands, information, signals, bits, symbols, and chips that may bereferenced throughout the above description may be represented byvoltages, currents, electromagnetic waves, magnetic fields or particles,optical fields or particles, or any combination thereof.

The various illustrative blocks and modules described in connection withthe disclosure herein may be implemented or performed with ageneral-purpose processor, a DSP, an ASIC, an FPGA or other programmablelogic device, discrete gate or transistor logic, discrete hardwarecomponents, or any combination thereof designed to perform the functionsdescribed herein. A general-purpose processor may be a microprocessor,but in the alternative, the processor may be any conventional processor,controller, microcontroller, or state machine. A processor may also beimplemented as a combination of computing devices (e.g., a combinationof a DSP and a microprocessor, multiple microprocessors, one or moremicroprocessors in conjunction with a DSP core, or any other suchconfiguration).

The functions described herein may be implemented in hardware, softwareexecuted by a processor, firmware, or any combination thereof. Ifimplemented in software executed by a processor, the functions may bestored on or transmitted over as one or more instructions or code on acomputer-readable medium. Other examples and implementations are withinthe scope of the disclosure and appended claims. For example, due to thenature of software, functions described above can be implemented usingsoftware executed by a processor, hardware, firmware, hardwiring, orcombinations of any of these. Features implementing functions may alsobe physically located at various positions, including being distributedsuch that portions of functions are implemented at different physicallocations. Also, as used herein, including in the claims, “or” as usedin a list of items (for example, a list of items prefaced by a phrasesuch as “at least one of” or “one or more of”) indicates an inclusivelist such that, for example, a list of [at least one of A, B, or C]means A or B or C or AB or AC or BC or ABC (i.e., A and B and C).

As those of some skill in this art will by now appreciate and dependingon the particular application at hand, many modifications, substitutionsand variations can be made in and to the materials, apparatus,configurations and methods of use of the devices of the presentdisclosure without departing from the spirit and scope thereof. In lightof this, the scope of the present disclosure should not be limited tothat of the particular embodiments illustrated and described herein, asthey are merely by way of some examples thereof, but rather, should befully commensurate with that of the claims appended hereafter and theirfunctional equivalents.

What is claimed is:
 1. A method of wireless communication, comprising:determining, at a user equipment (UE), channel correlation informationfor a channel between the UE and a base station; defining, at the UE, aperiodicity of transmission for a sounding reference signal (SRS) basedon the channel correlation information; and transmitting, from the UE,the SRS in accordance with the defined periodicity.
 2. The method ofclaim 1, wherein the transmitting the SRS includes transmitting during aportion of a subframe designated for SRS transmission.
 3. The method ofclaim 1, wherein the channel correlation information includes a velocityof the UE relative to the base station.
 4. The method of claim 1,wherein the periodicity is in the range from 2-4 subframes.
 5. A methodof wireless communication, comprising: determining, at a base station,channel correlation information for a channel between the base stationand a user equipment (UE); transmitting, from the base station, arequest for a sounding reference signal (SRS) from the UE based on thedetermined channel correlation information; receiving, at the basestation, the requested SRS; and training, at the base station,beamforming to the UE based on the received SRS.
 6. The method of claim5, wherein the channel correlation information includes an aperiodicdecrease in channel correlation.
 7. A method of wireless communication,comprising: determining, at a user equipment (UE), a processing gain(PG) to communicate with a base station; determining, at the UE, aminimum length of a sounding reference signal (SRS) based on thedetermined PG; and broadcasting, from the UE to the base station, an SRShaving at least the minimum length.
 8. The method of claim 7, whereinthe broadcasting the SRS includes broadcasting continuously across oneor more subframes.
 9. The method of claim 7, wherein the broadcastingthe SRS includes broadcasting during one or more subframes only during adesignated SRS portion of each subframe.
 10. The method of claim 7,wherein the determining the required PG includes monitoring a length oftime before the UE receives a SYNC signal from the base station.
 11. Amethod of wireless communication, comprising: determining, at a basestation, a processing gain (PG) to communicate with a user equipment(UE); determining, at the base station, a minimum length of a soundingreference signal (SRS) based on the determined PG; and transmitting,from the base station, a request to the UE for an SRS having at leastthe minimum length.
 12. The method of claim 11, wherein the transmittedrequest includes an instruction to broadcast the SRS continuously acrossone or more subframes.
 13. The method of claim 11, wherein thetransmitted request includes an instruction to broadcast the SRS duringone or more subframes only during a designated SRS portion of eachsubframe.
 14. The method of claim 11, wherein the determining therequired PG includes monitoring a length of time before successfullyestablishing a random access channel (RACH) with the UE.
 15. A userequipment (UE), comprising: a processor configured to: determine channelcorrelation information for a channel between the UE and a base stationand define a periodicity of transmission for a sounding reference signal(SRS) based on the channel correlation information; and/or determine aprocessing gain (PG) to communicate with the base station and a minimumlength of a sounding reference signal (SRS) based on the determined PG;and a transceiver in communication with the processor, the transceiverconfigured to: transmit the SRS to the base station based on thedetermined periodicity for the SRS and/or the determined minimum lengthof the SRS.
 16. The user equipment of claim 15, wherein the processor isconfigured to determine channel correlation information for a channelbetween the UE and the base station and define a periodicity oftransmission for a SRS based on the channel correlation information. 17.The user equipment of claim 16, wherein the SRS is transmitted during aportion of a subframe designated for SRS transmission.
 18. The userequipment of claim 16, wherein the channel correlation informationincludes a velocity of the UE relative to the base station.
 19. The userequipment of claim 16, wherein the periodicity is in the range from 2-4subframes.
 20. The user equipment of claim 15, wherein the processor isconfigured to determine a PG to communicate with the base station and aminimum length of a SRS based on the determined PG.
 21. The userequipment of claim 20, wherein the transceiver is further configured tobroadcast the SRS continuously across one or more subframes.
 22. Theuser equipment of claim 20, wherein the transceiver is furtherconfigured to broadcast the SRS during one or more subframes only duringa designated SRS portion of each subframe.
 23. The user equipment ofclaim 20, wherein the processor is further configured to monitor alength of time before the UE receives a SYNC signal from the basestation to determine the required PG.
 24. A base station, comprising: aprocessor configured to: determine channel correlation information for achannel between the base station and a user equipment (UE); and/ordetermine a processing gain (PG) to communicate with a user equipment(UE) and determine a minimum length of a sounding reference signal (SRS)based on the determined PG; and a transceiver in communication with theprocessor, the transceiver configured to: transmit a request for asounding reference signal (SRS) to the UE based on the determinedchannel correlation information and/or the determined minimum length ofthe SRS; and receive the requested SRS from the UE.
 25. The base stationof claim 24, wherein the base station is further configured to beamformto the UE based on the received SRS.
 26. The base station of claim 24,wherein the processor is configured to determine channel correlationinformation for a channel between the base station and a UE.
 27. Thebase station of claim 26, wherein the channel correlation informationincludes an aperiodic decrease in channel correlation.
 28. The basestation of claim 24, wherein the processor is configured to determine aPG to communicate with a UE and determine a minimum length of a SRSbased on the determined PG.
 29. The base station of claim 28, whereinthe transmitted request includes an instruction to broadcast the SRScontinuously across one or more subframes.
 30. The base station of claim28, wherein the transmitted request includes an instruction to broadcastthe SRS during one or more subframes only during a designated SRSportion of each subframe.
 31. The base station of claim 28, wherein theprocessor is further configured monitor a length of time beforesuccessfully establishing a random access channel (RACH) with the UE todetermine the required PG.
 32. A computer-readable medium having programcode recorded thereon, the program code comprising: code for causing auser equipment (UE) to determine channel correlation information for achannel between the UE and a base station and define a periodicity oftransmission for a sounding reference signal (SRS) based on the channelcorrelation information; and/or code for causing the user equipment (UE)to determine a processing gain (PG) to communicate with the base stationand a minimum length of the sounding reference signal (SRS) based on thedetermined PG; and code for causing the UE to transmit an SRS inaccordance with the defined periodicity and having at least the minimumlength.
 33. The computer-readable medium of claim 32, wherein thetransmitting the SRS includes transmitting during a portion of asubframe designated for SRS transmission.
 34. The computer-readablemedium of claim 32, wherein the program code comprises code for causingthe UE to determine channel correlation information for a channelbetween the UE and the base station and define a periodicity oftransmission for a SRS based on the channel correlation information. 35.The computer-readable medium of claim 34, wherein the channelcorrelation information includes a velocity of the UE relative to thebase station.
 36. The computer-readable medium of claim 34, wherein theperiodicity is in the range from 2-4 subframes.
 37. Thecomputer-readable medium of claim 34, further comprising: code forcausing the UE to broadcast continuously across one or more subframes.38. The computer-readable medium of claim 34, further comprising: codefor causing the UE to broadcast during one or more subframes only duringa designated SRS portion of each subframe.
 39. The computer-readablemedium of claim 32, wherein the program code comprises code for causingthe UE to determine a PG to communicate with the base station and aminimum length of the SRS based on the determined PG.
 40. Thecomputer-readable medium of claim 39, further comprising: code forcausing the UE to monitor a length of time before the UE receives a SYNCsignal from the base station.
 41. A computer-readable medium havingprogram code recorded thereon, the program code comprising: code forcausing a base station to determine channel correlation information fora channel between the base station and a user equipment (UE); and/orcode for causing the base station to determine a processing gain (PG) tocommunicate with the user equipment (UE) and determine a minimum lengthof the sounding reference signal (SRS) based on the determined PG; codefor causing the base station to transmit a request for a soundingreference signal (SRS) having the minimum length from the UE based onthe determined channel correlation information.
 42. Thecomputer-readable medium of claim 41, wherein the program code furthercomprises: code for causing the base station to receive the requestedSRS; and code for causing the base station to beamform to the UE basedon the received SRS.
 43. The computer-readable medium of claim 41,wherein the program code comprises code for causing the base station todetermine channel correlation information for a channel between the basestation and a UE.
 44. The computer-readable medium of claim 43, whereinthe channel correlation information includes an aperiodic decrease inchannel correlation.
 45. The computer-readable medium of claim 41,wherein the program code comprises code for causing the base station todetermine a PG to communicate with the UE and determine a minimum lengthof the SRS based on the determined PG.
 46. The computer-readable mediumof claim 45, wherein the transmitted request includes an instruction tobroadcast the SRS continuously across one or more subframes.
 47. Thecomputer-readable medium of claim 45, wherein the transmitted requestincludes an instruction to broadcast the SRS during one or moresubframes only during a designated SRS portion of each subframe.
 48. Thecomputer-readable medium of claim 45, further comprising: code forcausing the base station to monitor a length of time before successfullyestablishing a random access channel (RACH) with the UE.